Various industrial processes and equipment operate at very high pressures and temperatures. For example, throughout the world the industrial scale process for synthesizing urea involves the reaction of ammonia and carbon dioxide in large high-pressure reactors at temperatures in excess of 150° C. (302° F.) and pressures of approximately 150 bar (15.0 MPa). The process is well known and is described in, for example, U.S. Pat. Nos. 4,210,600, 4,899,813, 6,010,669, and 6,412,684. In the process, ammonia, which is generally in excess, and carbon dioxide are reacted in one or more reactors, obtaining as end products an aqueous solution containing urea, ammonium carbamate not transformed into urea, and the excess ammonia used in the synthesis.
The most corrosive conditions during urea synthesis occur when the ammonium carbamate is at its highest concentration and temperature. Although these conditions occur at the most critical step in the process, only relatively few materials can withstand the conditions without experiencing significant corrosion, which can lead to equipment failure. Materials from which urea synthesis equipment has been fabricated have included in part, over time, AISI 316L stainless steel, INOX 25/22/2 Cr/Ni/Mo stainless steel, lead, titanium, Safurex® stainless steel, and zirconium.
When the urea synthesis process was first developed, “urea grade” austenite-ferrite stainless steels and other proprietary grades of stainless steel were used. The synthesis equipment includes a stripper having a vertical tube bundle in which the urea process medium is decomposed and condensed. The urea process medium flows through the inner volume of the tubes, while saturated steam circulates and condenses on the outside of the tubes. The condensing steam provides the necessary energy to decompose the excess ammonia and ammonium carbamate within the tubes into urea and water. The spacing of the tubes in the stripper is maintained by tubesheets, which include circular holes through which the tubes pass, and the individual tubes also are joined to a surface of the tubesheets by strength welds.
Few materials can withstand the internal and external conditions to which the stripper tubes are subjected without experiencing significant corrosion and/or erosion over time. The corrosion resistance of stainless steels used in stripper tubes is largely dependent on whether the urea solution in the tubes is uniformly and evenly distributed on the tube surfaces so as to passivate the stainless steel (the solution provides a portion of the passivating oxygen). If the tubes' internal surfaces are not fully and continuously wetted, the stainless steel will corrode. Thus, if the processing unit is operated at a steady-state condition and at relatively high capacity, the stainless steel tubes will perform adequately. If the unit is operated at lower capacity, however, distribution of the urea process medium in the stripper tubes may be uneven or the tubes may include unwetted internal surfaces that are not totally passivated, resulting in corrosion. Thus, currently available stainless steels were not found to be reliable stripper tube materials for use in the urea synthesis process.
To address the corrosion problems experienced with stainless steels, over 30 years ago urea synthesis equipment fabricated from titanium was developed. In this design, the titanium-clad stripper includes solid titanium tubes joined to titanium-clad tubesheets. When this design was placed in service, the vertically disposed stripper tubes were subject to corrosion and erosion in a region in the vicinity of the strength welds fusing the tubes to the stripper tubesheets. Erosion and corrosion were also noted within the first 1 meter (39.4 inches) length of the tubes. The ammonium carbamate is at the highest concentration and temperature, and decomposes and condenses in this region, and it is postulated that the erosion/corrosion occurs because of the sudden change in fluid direction, fluid impingement, or sudden evaporation in this region. After the propensity for corrosion/erosion of titanium stripper tubes was identified, the equipment was redesigned so that the stripper units could be flipped end-to-end, thereby allowing for erosion/corrosion to occur on both ends of the stripper tubes before replacement of the tubes was necessary. Although this almost doubled the service life of the stripper tubes, it was not a permanent solution to the units' corrosion problem, and many of the urea processing units fabricated with titanium stripper tubes have experienced some degree of erosion/corrosion problems.
To further address the erosion and corrosion problems experienced in urea strippers, stripper tubes fabricated using zirconium were introduced, as described in U.S. Pat. No. 4,899,813. Because zirconium is more expensive than titanium and stainless steel, early zirconium-equipped stripper tubes were designed to include a stainless steel outer tube (generally 2 mm (0.8 inch) minimum thickness) and a relatively thin tubular inner liner of zirconium (generally 0.7 mm (0.03 inch) minimum thickness) mechanically bonded (snug fit) within the stainless steel tube. The mechanical bonding necessary to retain the zirconium liner in place was achieved by expanding the inner diameter of the zirconium liner so as to snugly fit within the stainless steel outer tube. The stainless steel outer tube of the resulting snug fit dual-layer tubing provides mechanical strength and also reduced the costs of the tubing relative to solid zirconium tubing. The relatively thin zirconium liner provides improved corrosion resistance. Zirconium was selected for this application because it exhibits excellent corrosion resistance in highly corrosive, high pressure, high temperature environments.
The foregoing stainless steel/zirconium snug fit dual-layer stripper tubing was manufactured under stringent requirements to better insure a very tight mechanical fit. Nevertheless, the mechanical bonding of the layers proved to be a source of trouble in tubes intended for long service lifetimes. Because of the absence of a metallurgical bond between the corrosion resistant zirconium liner and the stainless steel outer tube, a slight gap existed between the zirconium inner liner and the stainless steel outer tube. This gap, in part, resulted from the different mechanical and physical properties of zirconium and stainless steels. For example, the materials have very different thermal expansion coefficients and, when heated, stainless steel will expand to a greater degree than zirconium. Also, because of the dissimilar properties of the materials, they cannot be fusion welded together, and it became necessary to remove a portion of the zirconium liner from the stripper tube end in order to fusion weld the tube to the stainless steel tubesheets. Regardless of how well the stainless steel tubes and zirconium liners were fabricated and how tightly the tube components were mechanically fit together, it was found that over time corrosive urea process medium was able to infiltrate the small gap between the stainless steel and the zirconium, resulting in crevice corrosion and, finally, penetration of the stainless steel outer tube. In some urea strippers having this design, the tubes began to fail for this reason, requiring shutdown of the urea synthesis equipment to repair the problem, and resulting in substantial maintenance costs.
Yet another, recent development is a design for urea synthesis stripper tube bundles including solid zirconium stripper tubes, zirconium-clad tubesheets, and an explosive bonded zirconium cladding layer on all internal wetted surfaces. However, given the cost of urea synthesis equipment, it is typically less expensive to repair corroded parts of existing equipment than to replace the equipment with this new corrosion resistant design. While parts replacement may be a cost-effective option for stripper equipment including solid zirconium stripper tubes, zirconium-clad tubesheets, and zirconium cladding on wetted surfaces, it would be advantageous if titanium clad stripper units could be manufactured with stripper tubes having improved corrosion resistance. That is because titanium-clad stripper units tend to be significantly less expensive to manufacture than zirconium-clad units.
Accordingly, it would be advantageous to provide an improved design for stripper tubes of urea synthesis equipment. It also would be advantageous to provide a method of retrofitting existing strippers for urea synthesis equipment with a form of corrosion resistant replacement stripper tubes, while utilizing the strippers' existing tubesheets.
More generally, it would be advantageous to provide an improved design for and method for producing corrosion resistant fluid conducting parts for articles of equipment operating under conditions promoting corrosion. In addition to stripper units of urea synthesis equipment, such articles of equipment include, for example, other chemical processing equipment, condenser units, and heat exchanger equipment. It also would be advantageous to provide a method of retrofitting existing worn and/or corrosion-prone parts of equipment with corrosion resistant replacement parts, wherein the replacement parts are fabricated from corrosion resistant materials such as, for example, zirconium, zirconium alloys, titanium, titanium alloys, and stainless steels.